The invention relates to an apparatus and method for speech encoding, and to a speech recognition apparatus which contains such a speech encoding apparatus.
In an automatic speech recognition system, the acoustic features of an utterance to be recognized are measured, and are compared to acoustic models of each of a plurality of speech symbols, such as words or parts of words. In one known approach, the measured acoustic features of the utterance are encoded with acoustic prototypes from a library of acoustic prototypes, in order to simplify the comparison of the acoustic features of the utterance with the acoustic model of each word. Two known methods of encoding measured acoustic features with acoustic prototypes are described in an article entitled "Continuous Speech Recognition with Automatically Selected Acoustic Prototypes Obtained By Either Bootstrapping Or Clustering" by A. Nadas, et al (Proceedings ICASSP 1981, pages 1153-1155, April 1981).
The performance of the speech recognition system is dependent, in part, on the adequacy of the acoustic prototypes in the acoustic prototype library. This is especially true for continuous speech recognition tasks where coarticulation effects may be numerous.